This invention relates to the production of hydrocarbons from earth formations, and more particularly, to those hydrocarbon-bearing deposits where the oil viscosity and saturation are so high that insufficient steam injectivity can be obtained by current steam injection methods.
A very large resource of heavy oil and tar sands exists in the world, such as those in Alberta, Canada; Utah and California in the United States; the Orinoco Belt of Venezuela; and the USSR. The total world reserve of tar sand deposits is estimated to be 2,100 billion barrels of oil, of which about 980 billion are located in Alberta, Canada, and of which 18 billion barrels of oil are present in shallow deposits in the United States.
In the present art, heavy oil deposits are produced by steam injection to swell and lower the viscosity of the crude to the point where it can be pushed toward the production wells. If steam injectivity is high enough, this is a very efficient means of heating and producing the formation. However, a large number of reservoirs contain tar of sufficiently high viscosity and saturation that initial steam injectivity is severely limited, so that even with a number of "huff-and-puff" pressure cycles, very little steam can be injected into the deposit without exceeding the formation fracturing pressure. Most of these tar sand deposits have previously not been capable of economic production.
The most difficult problem in steamflooding deposits with low injectivity is establishing and maintaining a flow channel between injection and production wells. Several proposals have been made to provide horizontal wells or conduits within a tar sand deposit to deliver hot fluids such as steam into the deposit, thereby heating and reducing the viscosity of the bitumen in tar sands adjacent to the horizontal well or conduit. U.S. Pat. No. 3,986,557 discloses use of such a conduit with a perforated section to allow entry of steam into, and drainage of mobilized tar out of, the tar sand deposit. U.S. Pat. Nos. 3,994,340 and 4,037,658 disclose use of such conduits or wells simply to heat an adjacent portion of deposit, thereby allowing injection of steam into the mobilized portions of the tar sand deposit.
In an attempt to overcome the steam injectivity problem, several proposals have been made for various means of electrical or electromagnetic heating of tar sands. One category of such proposals has involved the placement of electrodes in conventional injection and production wells between which an electric current is passed to heat the formation and mobilize the tar. This concept is disclosed in U.S. Pat. Nos. 3,848,671 and 3,958,636. A similar concept has been presented by Towson at the Second International Conference on Heavy Crude and Tar Sand (UNITAR/UNDP Information Center, Caracas, Venezuela, September, 1982). A novel variation, employing aquifers above and below a viscous hydrocarbon-bearing formation, is disclosed in U.S. Pat. No. 4,612,988. In U.S. Reissue Pat. No. 30738, Bridges and Taflove disclose a system and method for in-situ heat processing of hydrocarbonaceous earth formations utilizing a plurality of elongated electrodes inserted in the formation and bounding a particular volume of a formation. A radio frequency electrical field is used to dielectrically heat the deposit. The electrode array is designed to generate uniform controlled heating throughout the bounded volume.
In U.S. Pat. No. 4,545,435, Bridges and Taflove again disclose a waveguide structure bounding a particular volume of earth formation. The waveguide is formed of rows of elongated electrodes in a "dense array" defined such that the spacing between rows is greater than the distance between electrodes in a row. In order to prevent vaporization of water at the electrodes, at least two adjacent rows of electrodes are kept at the same potential. The block of the formation between these equipotential rows is not heated electrically and acts as a heat sink for the electrodes. Electrical power is supplied at a relatively low frequency (60 Hz or below) and heating is by electric conduction rather than dielectric displacement currents. The temperature at the electrodes is controlled below the vaporization point of water to maintain an electrically conducting path between the electrodes and the formation. Again, the "dense array" of electrodes is designed to generate relatively uniform heating throughout the bounded volume.
Hiebert et al ("Numerical Simulation Results for the Electrical Heating of Athabasca Oil Sand Formations," Reservoir Engineering Journal, Society of Petroleum Engineers, January, 1986) focus on the effect of electrode placement on the electric heating process. They depict the oil or tar sand as a highly resistive material interspersed with conductive water sands and shale layers. Hiebert et al propose to use the adjacent cap and base rocks (relatively thick, conductive water sands and shales) as an extended electrode sandwich to uniformly heat the oil sand formation from above and below.
As can be seen from these examples, previous proposals have concentrated on achieving substantially uniform heating in a block of a formation so as to avoid overheating selected intervals. The common conception is that it is wasteful and uneconomic to generate nonuniform electric heating in the deposit. The electrode array utilized by prior inventors therefore bounds a particular volume of earth formation in order to achieve this uniform heating. However the process of uniformly heating a block of tar sands by electrical means is extremely uneconomic. Since conversion of fossil fuel energy to electrical power is only about 38 percent efficient, a significant energy loss occurs in heating an entire tar sand deposit with electrical energy.
We have discovered an economic method of selective heating particularly applicable to thick tar sand deposits containing thin, high conductivity layers. These thin conductive layers are typically shales into which the tar sand was alluvially deposited, but may also be water sands with or without salinity variations, or layers which also contain hydrocarbons but have significantly greater porosity. A thin conductive layer is heated to a temperature that is sufficient to form an adjacent thin preheated zone, in which the viscosity of the tar is reduced to a level sufficient to allow steam injection into the thin preheated zone. Electrical heating is then discontinued, and the deposit is steam flooded. The thin conductive layers to be heated are preferably in the lower portion of the tar sand deposit, and the electrically heated zones are typically only a small fraction of the total tar sand deposit. This localized heating generates a uniformly heated plane (the shale layer) within the tar sand deposit.
It is therefore an object of this invention to provide an efficient and economic method of in-situ heat processing of tar sand and other heavy oil deposits wherein electrical current is used to heat thin, highly conductive layers within such deposits, utilizing a minimum of electrical energy to prepare the tar sands for steam injection; and then to efficiently utilize steam injection to mobilize and recover a substantial portion of the heavy oil and tar contained in the deposit.